In recent years, new types of lasers using a two-dimensional photonic crystal have been developed. A two-dimensional photonic crystal consists of a plate-shaped dielectric base body with a periodic structure of refractive index formed therein. Typically, this device is created by providing the base body with a periodic arrangement of areas whose refractive index differs from that of the base body. (This area is hereinafter called the “modified refractive index area.”) This periodic structure causes a Bragg diffraction within the crystal and creates an energy band gap for the energy of light. There are two types of two-dimensional photonic crystal lasers: one type utilizes a band-gap effect to make a point defect function as a resonator, and the other type utilizes a standing wave at a band edge where the group velocity of light becomes zero. Each of these devices causes laser oscillation by amplifying light of a predetermined wavelength.
The latter type of the two-dimensional photonic crystal laser utilizing a standing wave has a layered structure in which a layer having a two-dimensional photonic crystal structure (this layer is hereinafter called the “two-dimensional photonic crystal layer”) is stacked on an active layer either directly or via another layer. These layers are sandwiched by other layers, such as a cladding layer for injecting electric charges into the active layer, a contact layer to be in contact with an external element, and a spacer layer for connecting these layers.
Patent Document 1 discloses a method of creating a two-dimensional photonic crystal laser by a process including the following steps: a structure including a cladding layer, spacer layer and other layers is prepared; this structure is stacked on a two-dimensional photonic crystal layer consisting of a base body with air holes (modified refractive index areas) periodically arranged therein, with the spacer layer being in contact with the two-dimensional photonic crystal layer; and the two-dimensional photonic crystal layer and the spacer layer are fused together by heat (thermal fusion bonding). In one example disclosed in Patent Document 1, both the base body of the two-dimensional photonic crystal layer and the spacer layer are made of n-type InP, and the heating temperature is 620° C. The layer stacked on the two-dimensional photonic crystal layer is hereinafter referred to as the “upper layer.”
Patent Document 2 discloses a method of creating an upper layer by epitaxially growing AlGaN directly on a two-dimensional photonic crystal layer consisting of a base body made of GaN with air holes periodically formed therein.
The methods described in Patent Document 2 can be broadly classified into the following three types: (i) A method in which a two-dimensional photonic crystal layer consisting of a base body with air holes as the modified refractive index areas periodically arranged therein is created, and then an upper layer is formed without filling the air holes; (ii) a method in which a layer including a base body with air holes periodically arranged therein is created, and then an upper layer is formed while filling the air holes to form modified refractive index areas; and (iii) a method in which modified refractive index areas in the form of columns are formed on a substrate, and then the spaces around them are filled by epitaxial growth to continuously form the base body and the upper layer.
In the case of the methods (ii) and (iii), the modified refractive index areas are made of a material other than air (more specifically, the same material as that of the upper layer). Such a structure has a lower light-confining effect as compared to the structure using air holes as the modified refractive index areas. However, the former structure is advantageous in that a single-mode laser oscillation can be more easily generated over a large area.